1. Field of the Disclosure
Embodiments disclosed herein relate generally to the processing of a C4 hydrocarbon cut from a cracking process, such as steam or fluid catalytic cracking. More specifically, embodiments disclosed herein relate to the separation and recovery of isobutene from a C4 hydrocarbon cut, where the resulting C4 fractions, which may separately include 2-butene, 1-butene, and/or isobutene, may be used in subsequent alkylation, oligomerization, etherification, dehydrogenation, and metathesis processes.
2. Background
In typical olefin plants, such as illustrated in U.S. Pat. No. 7,223,895, there is a front-end demethanizer for the removal of methane and hydrogen followed by a deethanizer for the removal of ethane, ethylene and C2 acetylene. The bottoms from this deethanizer tower consist of a mixture of compounds ranging in carbon number from C3 to C6. This mixture may be separated into different carbon numbers, typically by fractionation.
The C3 cut, primarily propylene, is removed as product and is ultimately used for the production of polypropylene or for chemical synthesis such as propylene oxide, cumene, or acrylonitrile. The methyl acetylene and propadiene (MAPD) impurities must be removed either by fractionation or hydrogenation. Hydrogenation is preferred since some of these highly unsaturated C3 compounds end up as propylene thereby increasing the yield.
The C4 cut consisting of C4 acetylenes, butadienes, iso- and normal butenes, and iso- and normal butane can be processed in many ways. A typical steam cracker C4 cut contains the following components in weight %:
TABLE 1Typical C4 cut components and weight percentages.C4 AcetylenesTraceButadienes33%1-butene15%2-butene 9%Isobutene30%Iso- and Normal Butanes13%
Typically, the butadiene and C4 acetylenes are removed first. This can be accomplished by either hydrogenation or extraction. If extraction is employed, the remaining 1-butene and 2-butene remain essentially in the same ratio as that of the initial feedstock. If hydrogenation is employed, the initial product from butadiene hydrogenation is 1-butene. Subsequently, hydroisomerization occurs within the same reaction system converting the 1-butene to 2-butene. The extent of this reaction depends upon catalyst and reaction conditions within the hydrogenation system. However, it is common practice to limit the extent of hydroisomerization in order to avoid “over hydrogenation” and the production of butanes from butenes. This would represent a loss of butene feedstock for downstream operations. The butenes remaining in the mixture consist of normal olefins (1-butene, 2-butene) and iso-olefin (isobutene). The balance of the mixture consists of both iso- and normal-butanes from the original feed plus what was produced in the hydrogenation steps and any small quantity of unconverted or unrecovered butadiene.
The butenes have many uses. One such use is for the production of propylene via metathesis. Another is for the production of ethylene and hexene via metathesis. Conventional metathesis involves the reaction of normal butenes (both 1-butene and 2-butene) with ethylene. These reactions occur in the presence of a group VIA or VIIA metal oxide catalyst, either supported or unsupported. Various metathesis processes are disclosed in, for example, U.S. Pat. Nos. 6,683,019, 6,580,009, 6,271,430, 6,777,582, and 6,727,396.
In some cases, an isobutene removal step is employed prior to metathesis. Options include reacting it with methanol to produce methyl tertiary butyl ether (MTBE) or separating the isobutene from the butenes by fractionation. U.S. Pat. No. 6,358,482 discloses the removal of isobutene from the C4 mixture prior to metathesis. This scheme is further reflected in U.S. Pat. Nos. 6,075,173 and 5,898,091.
Isobutene removal from the C4 stream can also be accomplished by employing a combined catalytic distillation hydroisomerization deisobutenizer system to both remove the isobutene and recover n-butenes at high efficiency by isomerizing the 1-butene to 2-butene with known isomerization catalysts, thus increasing the volatility difference. This technology combines conventional fractionation for isobutene removal with hydroisomerization within a catalytic distillation tower. In U.S. Pat. No. 5,087,780 to Arganbright, 2-butene is hydroisomerized to 1-butene as the fractionation occurs. This allows greater than equilibrium amounts of 1-butene to be formed as the mixture is separated. Similarly, 1-butene can be hydroisomerized to 2-butene in a catalytic distillation tower. In separating a C4 stream containing isobutene, 1-butene, and 2-butene (plus paraffins), it is difficult to separate isobutene from 1-butene since their boiling points are very close. By employing simultaneous hydroisomerization of the 1-butene to 2-butene with fractionation of isobutene, isobutene can be separated from the normal butenes at high efficiency.
In U.S. Pat. No. 7,214,841, for example, the C4 cut from a hydrocarbon cracking process is first subjected to auto-metathesis prior to any isobutene removal and without any ethylene addition, favoring the reactions which produce propylene and pentenes. The ethylene and propylene produced are then removed leaving a stream of the C.sub.4's and heavier components. The C5 and heavier components are then removed leaving a mixture of 1-butene, 2-butene, isobutene, and iso- and normal butanes. The isobutene is next removed preferably by a catalytic distillation hydroisomerization de-isobutenizer. The isobutene-free C4 stream is then mixed with the product ethylene removed from the auto-metathesis product together with any fresh external ethylene needed and subjected to conventional metathesis producing additional propylene.
In the above processes, separation of isobutene from normal butenes may be accomplished via isomerization of 1-butene to 2-butene, facilitating the fractionation of the normal butene from isobutene. The continuous fractionation of 2-butenes away from the reaction zone improves the driving force of the isomerization to 2-butenes. The resulting products can achieve conversion beyond the equilibrium ratio of the reaction. Unfortunately, due to high reflux requirements, these processes use large amounts of utilities, such as cooling water and steam.
Accordingly, there exists a significant need for C4 separation processes that may provide the desired separations at reduced capital cost and/or utility consumptions.